Advanced Biochemistry - Metabolism 45 questions assignment

Fundamentals of Biochemistry

Fourth Edition

Chapter 19 Photosynthesis

Donald Voet • Judith G. Voet • Charlotte W. Pratt

Synthase vs Synthetase As I learned more molecules I started seeing many enzymes interchange the words Synthase and Synthetase for enzymes that synthesize or create molecules. The difference between synthase and synthetase are simple. Synthetases use a high energy Nucleoside Triosphosphate such as Adenosine Triosphosphate (ATP) or Guanosine Triosphosphate (GTP) while Synthases do not.

hCp://www.dbriers.com/tutorials/2011/10/the-difference-between-the-enzymes- synthase-and-synthetase/

Chapter 19 Chloroplasts

Key Concepts 19.1 • The chloroplast thylakoid membrane is the site of light absorpLon. • Pigment molecules, some arranged in light-harvesLng complexes, absorb visible light.

Chloroplast From Corn

Stroma is equivalent to mitochondrial Matrix space Thylakoids are equivalent to mitochondrial Christae Thylakoid compartment (aka Lumen) is equivalent to inner membrane space

Chloroplast From Corn

Chloroplast From Corn

Chlorophyll Structures

Absorption Spectra of Photosynthetic Pigments

Lower energy quanta

Higher energy quanta

Energy Flow Through Photosynthetic Antenna Complex

Light-Harvesting Complex

Rs. molischianum PDBid 1LGH

Light-Harvesting Complex

Rs. molischianum PDBid 1LGH

Accessory Pigments “Fill In” the Absorption Spectra of Antenna Complex

Accessory Pigments Used by Water- Dwelling Photosynthetic Organisms

Chapter 19 Chloroplasts

Checkpoint 19.1 • Summarize the events of the light reacLons and light-independent reacLons. • Describe the structure of the chloroplast. • Why do photosyntheLc organisms contain several types of pigment molecules? • What is the funcLon of light-harvesLng complexes?

Chapter 19 The Light Reactions

Key Concepts 19.2 • Absorbed light energy can be dissipated by internal conversion, fluorescence, exciton transfer, or photooxidaLon. • The special pair of the purple bacterial photosyntheLc reacLon center undergoes photooxidaLon, and an electron-transport chain returns an electron to the special pair. • In plants and in cyanobacteria, two photosystems, cytochrome b6f, and mobile electron carriers form an electron-transport chain described by the Z-scheme. • Photosystem II reduces its photooxidized special pair with electrons derived from water.

Chapter 19 The Light Reactions

Key Concepts 19.2 • Electrons traveling from photosystem II through the cytochrome b6f complex undergo a Q cycle that generates a transmembrane proton gradient. • Electrons liberated by photooxidaLon of photosystem I reduce NADP+ or return to the cytochrome b6f complex, whose acLvity contributes to the proton gradient. • ATP is produced by photophosphorylaLon.

Electronic States of Chlorophyll & Modes of Interconversion

Excitation Energy Trapping

Photosynthetic Reaction Center

Rb. sphaeroides PDBid 2RCR

PbRC Electron Acceptor

Disposition of Prosthetic Groups in Photosynthetic Reaction Center

Electron-Transport System of Purple Photosynthetic Bacteria

Model of Thylakoid Membrane

Coupling of Electron Transport and ATP Synthesis

Chloroplast Mitochonrion

4 photons per O2 produced (quantum yield)

P/O raFo 2.5

PSI, PSII Complex I, Complex II Complex III Complex IV

ATPsynthetase faces Stroma

ATPsynthetase faces Matrix

Q cycle connects PSI and PSII

Q cycle connects Complex I, II and Complex III

pH Gradient 4.5 in lumen, 8.0 in Stroma

pH Gradient 5 in inner membrane, 7.5 in Matrix

Electron flow from H2O to NAD (produces O2)

Electron flow from NADH to O2 (produces H2O)

DCMU Blocks Electron Flow From PSII to PSI

Ubiquinone Analog Links PSII to Cytochrome b6f

Z-Scheme of Photosynthesis

But Paul, I don’t see a “Z”

Why don’t they call it the “N- Scheme”???

The Bendall-Hill Z Scheme (by Govindjee)

WileyVCH 2012

UV-vis spectrum of Cytochrome f in Reduced and Oxidized states

(Ho & Krogmann, JBC 255:3855-61, 1980)

reduced

oxid

When Chloroplast illuminated with 700 nm light, oxidized form; When illuminated with 650 nm light, reduced form is seen

Emerson’s “Red Drop” When wavelength of light > 680 nm, the O2/quantum drops

(even though chlorophyll absorbs significantly at those wavelengths)??????

Absorption Spectra of Photosynthetic Pigments

Lower energy quanta

Higher energy quanta

Cytochrome f redox state depends on wavelength “Red Drop” in O2/quantum of light when light is > 680 nm MAYBE 2 light reacLons!!! One opLmal at 650 nm, One opLmal at 700 nm Maybe cyt f sits BETWEEN the two light reacLon centers!! Above 650 nm, the light does not have enough energy to excite the reacLon center necessary for O2 evoluLon

VIOLA! 2 light reacLons!!!

Plant photosyn has a Q cycle that connects two complexes (PSI and PSII). Analogous to Complexes of Mitochondrial e- transport. Generates H+ gradient

Z-Scheme of Photosynthesis

Cyclic Electron Flow in Plants Involves only PSI Electrons from excited ChlA go to Ferredoxin and then to Q cycle, cytb/f, back to Chl No NET electron transfer, No NADP reducLon BUT H+ translocaLon to drive ATP synthesis Non-Cyclic Electron Flow in Plants Involves PSI and PSII Electrons from excited ChlA go to Ferredoxin then to NADP via Fd:NADP oxidoreductase Oxidized ChlA re-reduced by Plastocyanin, which receives electrons from cytb/f, which received electrons from Q, which was reduced by PSII PSII ulLmately receives electrons from H2O (Mn-Water Spliung Complex)

PSII Resembles PbRC

T. elongatus PDBid 1S5L

Transmembrane Portions of PSII Complex

T. elongatus PDBid 1S5L

Electron-Transfer Cofactors in PSII

T. elongatus PDBid 1S5L

Periodic O2 Evolution Suggests Multiple Light-Dependent Reactions

5-Stage Mechanism of O2 Generation in Chloroplasts

Model of OEC (The Oxygen Evolving Complex)

W = Water molecule

Cytochrome b6f Complex Generates Proton Gradient

Mas7gocladus laminosus PDBid 1UM3

Plastocyanin Transports Electrons From Cytochrome b6f to PSI

Plastocyanin (PC) PDBid 1PLC

PSI Resembles PSII & PbRC PbRC = photosynthetic bacteria reaction center

Single Protomer of PSI

Cofactors of PSI RC & PsaC

PSI RC and PsaC PDBid 1JB0

PSI’s RC Contains Two Phylloquinone Molecules with Phytyl Side Chain

Most PSI-Activated Electrons Reduce Ferredoxin

Peptococcus aerogenes PDBid 1FDX

Ferredoxin-NADP+ Reductase Mediates Fd-Dependent Reduction of NADP+

Box 19-1: Segregation of PSI & PSII

Grana Stacks have both PSI and PSII; Unstacked Stoma Lamellae have only PSI ATP Synthetase complex found in both

CF1 “Lollipops” of ATP Synthases in Thylakoid

Chapter 19 The Light Reactions

Checkpoint 19.2 • How do molecules dissipate absorbed light energy? Which mechanism is most important for photosynthesis? • Describe the events that occur awer the special pair of PbRC absorbs a photon. • Explain why electron transport in purple photosyntheLc bacteria follows a circular path. • Describe the effect of photooxidaLon on the redox reacLons summarized in the Z-scheme. • What is the importance of the water-spliung reacLon for photosynthesis?

Chapter 19 The Light Reactions

Checkpoint 19.2 • What does the Q cycle accomplish in bacterial and plant photosyntheLc electron transport? • What chloroplast protein is the funcLonal counterpart of mitochondrial cytochrome c? • What are the implicaLons of cyclic and noncyclic electron transfer in PSI? • Compare and contrast photophosphorylaLon and oxidaLve phosphorylaLon. • Explain how the energy of one photon is transformed to the energy of one ATP. • What is the relaLonship between the number of photons absorbed and the amount of O2produced?

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